1. Field of the Invention
This invention relates generally to variable pitch, flat-plate blade rows for turbomachinery, and more particularly, to variable pitch, offset multiple flat-plate blade rows for bidirectional thrust or flow devices which effectively provide variable camber blade row configurations.
2. Description of the Prior Art
Variable pitch, flat-plate blade rows are commonly used in applications where bidirectional thrust or flow is required. One such application is bow or stern bidirectional thrusters installed on naval ships to provide a turning capability independent of the ship's main propulsion and steering systems. Another application is in certain blower and air distribution duct systems where bidirectional air flow is required.
Conventional bidirectional variable pitch flat-blade row thrust or flow devices are low rotational speed units which are large and heavy. Such devices may be driven at constant or variable rotational speeds, but the direction of rotation need not be reversed to produce bidirectional thrust or flow, thereby simplifying the power transmission system thereof. A flat-plate or uncambered geometry of the blade rows provides the capability of reversing the direction of the thrust or flow without reversing the direction of rotation of the mounting hub holding the blade rows. This flat-plate or uncambered blade geometry also ensures that substantially equivalent performance characteristics, i.e., thrust or flow, are produced in either direction. The measure of useful work output, i.e., thrust or flow, produced by conventional bidirectional thrust or flow devices is determined by the total turning angle through which the working fluid flow is directed. Flat-plate or uncambered blade rows are dependent solely upon the incidence angle of the approaching relative fluid velocity to generate this total angle since the blade curvature of the rows is zero, i.e., uncambered, with larger relative working fluid velocity incidence angles required to generate greater total turning angles of the working fluid to increase the performance of these conventional bidirectional thrust or flow devices. The incidence angle of the approaching relative fluid velocity is in turn determined by the pitch angle setting of the flat-plate or uncambered blade rows. The efficiency of conventional bidirectional variable pitch flat-plate blade row thrust or flow devices, however, is limited by the flat-plate geometry of the blade rows. Conventional designs for such devices have generally been limited to single-row, lightly loaded blades having limited pitch angle setting ranges since cavitation, i.e., the phenomenon wherein vapor bubbles are formed at the inlet of the device due to the net positive suction head being less than the vapor pressure of the fluid, and subsequently collapse violently at some point downstream of the inlet, producing very sharp, crackling noises, frequently accompanied by physical damage to adjacent metal surfaces, and flow separation, i.e., the phenomenon wherein flow path of the working fluid separates from the uncambered blade surface, impose limiting restrictions on allowable flow incidence angles of the relative fluid flow.
A conventional, bidirectional pump, blower, or fan 10 design comprising a variable pitch, single flat-plate or uncambered blade row 12 is depicted in FIGS. 1(a) and 1(b) and 1(c). The variable pitch, single blade row 12 is comprised of a plurality of flat-plate or uncambered blades 14 mounted along the centerline of rotatable hub 16 by pivotal attaching means 18. These elements are rotatably disposed within a casing or duct 20 by means of nonflow interfering stationary hubs 22 rigidly secured to casing 20 by guide vane struts 24. These guide vane struts 24 also function to direct the working fluid flow 26a or 26b onto the plurality of uncambered blades 14 of the variable pitch single blade row 12. A rotating mechanism (not shown) cooperates with the rotatable hub 16 for rotation thereof, as shown in FIG. 1(a) in the direction of arrow 28.
The modulation of the thrust or flow rate developed by this conventional, bidirectional pump, blower, or fan 10, either in direction or absolute magnitude, without changing the direction of rotation of the rotatable hub 16 or the rotational speed thereof, is accomplished by varying the pitch angle .theta. of the plurality of uncambered blades 14 about the pivotal attaching means 18. A hub-mounted adjustment mechanism (not shown) cooperates with the pivotal attaching means 18 of each of the plurality of uncambered blades 14 to vary the pitch angle .theta. thereof, thereby varying the magnitude of the thrust of flow, and/or the direction thereof. The settings of the pitch angle .theta. of the plurality uncambered blades 14 is referenced to a tangential direction 30, which is parallel to the direction of rotation 28 and perpendicular to the direction of the working fluid flow 26a or 26b, and a chordline 29 of the uncambered blades 14. The pitch angle .theta. is conventionally varied between zero degrees, i.e., the chordlines 29 of the plurality of uncambered blades 14 lying parallel to the tangential direction 30, a minimum (negative) pitch angle -.theta. as depicted in FIG. 1(b), and a maximum (positive) pitch angle +.theta. as depicted in FIG. 1(c). When the pitch angle .theta. of the plurality of uncambered blades 14 is set at zero degrees, no thrust of flow is produced by the conventional bidirectional pump, blower, or fan 10. When the pitch angle .theta. is set at -.theta. or +.theta., depending upon the direction of the working fluid flow 26a or 26b, maximum thrust or fluid flow is produced. Varying the setting of pitch angle .theta. of the plurality of uncambered blades 14 between zero degrees and the minimum (negative) pitch angle -.theta. (or the maximum (positive) pitch angle +.theta.) by means of the hub-mounted adjustment mechanism (not shown), the magnitude of the thrust of fluid flow in a given direction is modulated without changing the rotational speed of the rotatable hub 16. Likewise, by varying the pitch angle .theta. of the plurality of uncambered blades 14 to crossover from a positive pitch angle to a negative pitch angle, or vice versa, the direction of the thrust or flow develop by the conventional bidirectional pump, blower, or fan 10 is reversed. These conventional bidirectional pumps, blowers, and fans, however, are limited as to the maximum (positive) pitch angle +.theta. and minimum (negative) pitch angle -.theta. at which the plurality of uncambered blades 14 may be set, and hence in the magnitude of thrust or fluid flow developed thereby, because once the maximum or minimum pitch angle setting is exceeded, cavitation and/or flow separation occurs and pump 10 performance deteriorates.
The prior art is replete with disclosures of blade row designs for use with thrust or flow devices. Representative references are U.S. Pat. Nos. 3,986,794 to Korn, 3,946,554 to Neumann, 3,442,493 to Smith. Jr., 2,805,818 to Ferri, and 2,314,572 to Chitz (Sheets). Smith, Jr. teaches static, cambered rotor vanes, Chitz teaches cambered blades having slots therein, Ferri teaches cambered rotor vanes succeeded by rotatable, cambered stator blades, Neumann teaches pitch adjustable cambered fan blades, and Korn teaches pitch adjustable cambered vanes. Each of these references, however, suffers from the inherent deficiency of cambered vanes so that even though the vanes may be pitch adjusted, the camber of the vanes causes unequal thrust or fluid flow performance characteristics at varied pitch settings, therefore making the vanes and blades as disclosed by these references inappropriate for bidirectional pumps, blowers, or fans. Korn discloses cambered vanes which can be pitch adjusted for forward thrust in a manner such that a vane from a second row abuts a succeeding vane in a third row to form a first cambered vane, and these vanes can also be pitch adjusted for reverse thrust in a manner such that the same vane from the second row abuts a succeeding vane in a first row to form a second cambered vane. Even though Korn discloses a mechanism which can form cambered vane rows for producing forward and reverse thrust, the camber of these vanes in the forward and reverse configurations is significantly different, thereby producing different thrust forces in the forward and reverse configurations. Moreover, the reverse thrust configuration of the vanes is only intended to be operational for short durations during engine operation, approximately 1% or less of the total engine operating time. As discussed hereinabove, bidirectional thrust or flow devices must be capable of developing substantially equivalent thrust or flow performance characteristics in either direction, inasmuch as an effective bow or stern bidirectional thruster, for example, must be capable of effectively turning a ship either direction with equal facility.